Guy 2: "She used to bang Claude. In fact, Claude and Javier are Milk Brothers"
The phrase originally referred to unrelated children who were breastfed by the same woman, symbolically becoming brothers, by sharing in this motherly attention.
The children became symbolic brothers by suckling at the same breast, hence the men become symbolic brothers through a similar, but less nutritious, action.
"Maria is such a slut, half the guys in town are milk brothers now."